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PHOENIX — A U.S. Border Patrol agent was charged Monday with first-degree murder for shooting an illegal immigrant during a confrontation on the Arizona side of the border in January.

An investigation found that Agent Nicholas Corbett's killing of Francisco Dominguez-Rivera of Puebla, Mexico, was not legally justified, said Cochise County's top prosecutor, Ed Rheinheimer.

"We have concluded that the evidence shows that at the time he was shot, Mr. Dominguez-Rivera presented no threat to Agent Corbett," Rheinheimer said.

Corbett was named in a criminal complaint that also charges him with second-degree murder, manslaughter and negligent homicide. Rheinheimer said that a judge will determine during a preliminary hearing which of the charges is best supported by the evidence.

Corbett's attorney, Daniel Santander, did not immediately return a message left Monday afternoon by The Associated Press.

The shooting, which drew condemnation from the Mexican government, occurred while Corbett was trying to apprehend Dominguez-Rivera and three others who were trying to enter the country illegally.

In the days that followed the incident, the Border Patrol said a scuffle had led to the shooting and the agent "feared for his life."

More than 300 pages of documents later released by Rheinheimer's office revealed that Corbett's account of what led him to shoot and kill the unarmed Dominguez-Rivera did not match witness testimony or forensic evidence.

Corbett declined to be interviewed by investigators but told other agents — whose accounts were part of the investigative file — that he came around the front of his sport utility vehicle, saw a man with a rock in his hand close to the rear of the vehicle and fired once when the man made a motion to throw it.

But three witnesses who were being apprehended along with Dominguez-Rivera — his two brothers and a sister-in-law — told investigators Corbett fired while pushing Dominguez-Rivera to the ground.

Rheinheimer said the evidence in the case corroborates the description given by the three witnesses.

Corbett remains a Border Patrol agent although he is not working in the field, said Gus Soto, a Border Patrol spokesman in Tucson.

Soto said the agency would make a statement on the charges on Tuesday.